Solvent dependence of the kinetics of formation and dissociation of cryptate complexes

Efficiency, Na+/K+ selectivity and temperature dependence of ion transport through lipid membranes by (221)C10-Cryptand, an ionizable mobile carrier

“…The rate constants for the ionization and protonation of the amine groups of the cryptand and also for the alkali cation complexation in its cavity were shown to be very high (Pizer, 1978;Cox et al, 1981). Conversely, these constants for cation-carrier decomplexation in water were rather low, i.e.…”

“…Conversely, these constants for cation-carrier decomplexation in water were rather low, i.e. 14.5 sec -z for Na+-(221) and 7.5 sec -1 for K+-(222) at 25~ (Cox et al, 1981). Consequently, only the activation energy for the decomplexation process might have contributed significantly to the energy involved in the overall transport process, especially in the case of K + transport by (222)C~0.…”

“…mole -1) was higher than that for (222)C~0 (29.5 kcal 9 mole-l). The following factors might account for this difference: i) the slightly different interactions between either Na+-(221)Cj0 or (Ginsburg & Stark, 1976); ii) the different temperature-dependences of Na + and K + ion interactions with the surface membrane (Simon et al, 1975); iii) the lower rate constant for the dissociation of K+-(222)CL0 complexes at the internal interface than for that of Na+-(221)C10 complexes based on the rates observed for the (221)-and (222)-cryptand homologues themselves (Cox et al, 1981); and iv) the fact that a higher true translocation rate constant was expected for K+-(222)C10 complexes than for Na+-(221)C10 complexes, because the (222)C10 concentration used was 5 times lower than the (221)C~0 concentration. Consequently, the electrical repulsion effect among the complexes in the lipophilic region of the membrane was also weaker for cation transport by (222)C10 than by (221)C10.…”

On the Nature of the Transition State Characterizing Gated Molecular Encapsulations

“…Accordingly, for the complexation of metal cations with crown ethers, Schneider and Cox reported that the rates by which cations access (k in ) the macrocycles are fast (approaching a diffusion-controlled limit) while their departure from the complex (k out ) is much slower corresponding to the thermodynamic stability (∆G°) of the complex OPEN ACCESS itself [10]. Importantly, the corresponding mechanistic study [11] indicated an early transition state for the entrapment, resembling reactants to account for the absence of the selectivity in complexation events. For gated molecular encapsulations [12], however, a conformational change in a gated host creates an aperture permitting in/out exchange of guests.…”

Ligand Exchange Processes on Solvated Lithium Cations: Complexation by Cryptands in Acetone as Solvent

“…[13] In case of crown ethers and cryptands in non-aqueous solutions, the rates of complex-formation (k f ) with alkalimetal cations are in general diffusion-controlled and, consequently, the complexation selectivities are governed by the decomplexation rates (k d ). [15] These reactions have received significant attention, initially from Lehn and co-workers and more recently from Popov et al [16,17] and Lincoln et al [18] Many examples have been included in a series of review articles. [19,20] It has been reported that ligand structure and solvent properties have considerable effects on the reaction rates, activation parameters, and exchange mechanism of cations between the solvated and complexed sites.…”